A ballistic resistant sheet comprises a stack of at least 4 monolayers, each monolayer containing unidirectionally oriented reinforcing fibers with at most 20 mass % of a matrix material, and with the fiber direction in each monolayer being rotated with respect to the fiber direction in an adjacent monolayer. Such a ballistic resistant sheet is very suitable for use in compressed or moulded ballistic resistant articles such as panels and especially curved panels.
Such a ballistic resistant sheet is known from U.S. Pat. No. 4,623,574. This publication discloses the manufacture of ballistic resistant sheets by cross plying and stacking a plurality of monolayers, each with unidirectionally aligned extended chain polyethylene fibers and a matrix material, followed by pressing them into a sheet. Example 1 of this disclosure mentions the production of a monolayer by helically wrapping polyethylene fibers side-by-side on a drum winder whereby a Kraton D1107 solution was used to coat the unidirectionally aligned fibers. A plurality of the thus obtained monolayers was stacked whereby the fiber direction in a monolayer is perpendicular to the fiber direction in an adjacent monolayer. The obtained stack was put between parallel plates in an Apollo press and pressed with a pressure of 0.6 MPa at a temperature of 130° C. for 5 minutes, followed by cooling.
There is continuous drive towards improved ballistic resistant moulded articles and the present inventors have surprisingly found a ballistic resistant sheet that enables the manufacture of compressed panels or ballistic resistant moulded articles with improved mouldability. Improved mouldability means that upon moulding of a ballistic resistant article, especially a curved ballistic resistant article, comprising several ballistic resistant sheets of the invention a homogeneous product is obtained; this can be judged by the human eye e.g. by absence of an inhomogeneous drape of the ballistic resistant sheets in said article after moulding.
According to the present invention an improved ballistic sheet is provided, comprising a stack of at least 4 monolayers, each monolayer containing unidirectionally oriented reinforcing fibers with a tensile strength of between 3.5 an 4.5 GPa, and at most 20 mass % of a matrix material, the areal density of a monolayer of at least 25 g/m2 and with the fiber direction in each monolayer being rotated with respect to the fiber direction in an adjacent monolayer.